When a remote sends an IR signal, the pattern of light flashes is generated by a subroutine, termed the executor. The remote reacts to a button press by routing control to the particular executor associated with the setup code along with the EFC data.

A long time ago, executors were pretty simple, components only responded to a single device number and IR signals used 8 bits of function data. That meant that 3 digit EFCs were sufficient to provide the necessary data to the executor.  The device number was encoded in a byte of Fixed Data, which is the same for all button presses.

Now many components respond to several device numbers and/or subdevice numbers. Some IR protocols use 16 bits of function data. So UEI switched to 5 digit EFCs. The 5 digits may be used to describe 8 bit function data in conjunction with multiple device/subdevice numbers (e.g. Sony), or a single device number combined with 16 bits of function data, or some intermediate combination.  There are usually several bytes of Fixed Data, describing the various device numbers, or other instructions to the executor.

There still are a few older IR protocols for which a 3 digit EFC suffices, and those are represented in a 5 digit system by prepending 2 zeros.

To get EFC numbers for Sony equipment, use three tools.
hifi-remote.com/sony/  Gives you all the device numbers and function numbers, as well as the type of Sony protocol 12, 15, or 20 bits.  But with your remote, you'll probably be using a 12/15/20 Combo executor.
www.hifi-remote.com/jp1/lookup/.  The Lookup Tool.  Allows you to find the setup codes which enable a given IR protocol, and to find the stock EFCs for that setup code.
www.hifi-remote.com/forums/viewtopic.php.  RemoteMaster.  Download this free software, select your remote, enter in the desired protocol (Sony 12/15/20) and match the indicated device/subdevice numbers from the Lookup Tool (probably setup code 1658), check that the Fixed Data sections match, and on the Functions tab, enter in the function numbers.  (For Sony 12/15/20, subdevice numbers are listed on the Setup tab, while the main device numbers are entered on the Functions tab).  The EFC column will reflect your choices, and that's the EFC you need to enter into the remote.